This invention relates to a modulated signal transmission system which is capable of adaptively compensating for nonlinearity of a power amplifier in a baseband.
With a development of high efficiency digital transmission, a high precision and a multi-level have been required in connection with modulation and demodulation. For example, on multilevel quadrature amplitude modulation in radio communication, it is very important in a transmission system to prevent an increase of a nonlinear distortion which might occur in a power amplifier. In other words, the power amplifier should have a linear amplification characteristic.
To this end, it is usual that the power amplifier is formed by a class A amplifier which has excellent linearity. However, use of such a class A amplifier results in a reduction of power efficiency.
On the other hand, a class B amplifier is often used as the power amplifier in the transmission system. As well known in the art, the class B amplifier is good in power efficiency but is inferior in linearity to the class A amplifier. Taking this into consideration, a linearizer which wall be often called a nonlinear compensator or a nonlinear converter is combined with the class B amplifier so as to compensate for nonlinearity of the class B amplifier. The combination of the class B amplifier and the linearizer enables improvement of not only the power efficiency but also the nonlinearity of the class B amplifier.
More specifically, the linearizer has a nonlinear conversion characteristic adaptively controlled and is operable in response to both an input signal of a baseband and a linearly detected signal which is obtained by linearly detecting a modulated signal sent from the power amplifier. The linearizer compares the input signal with the linearly detected signal to adaptively control the nonlinear conversion characteristic.
With this structure, it is possible to favorably compensate for a nonlinear distortion included in the modulated signal sent from the power amplifier. However, it has been found out that the linearizer can not completely compensate for a linear distortion. This is because no distinction between the nonlinear and the linear distortions is made in the linearizer and the nonlinear distortion alone is compensated by the linearizer.